The clang of steel on steel echoes through the forest as entomologist Jose Negron hammers a chisel into a dead lodgepole pine and peels back the bark. Less than a quarter-inch beneath the scaly, gray bark, snug in a layer of dead brown wood, is an off-white, wormlike insect smaller than a grain of rice.

“There’s a nice plump one,” said Negron, prodding it with the tip of a knife blade. “That one’s still alive, I think, because it’s still nice and soft and white.”

The tiny grub—a mountain pine beetle larva—looks harmless.

But the fact that it’s still alive, apparently unfazed by temperatures that hit minus-24 degrees Fahrenheit in January at one of Negron’s experimental sites near Winter Park, makes this little bug a huge threat to Colorado forests.

“Cold-hardiness” is one of the traits that’s helping the mountain pine beetle perpetuate the epidemic now decimating the state’s lodgepole forests.

More than 640,000 acres of Colorado lodgepoles—42 percent of the state’s total—were infested last year, according to the Colorado State Forest Service.

Countless larvae now “overwintering” inside lodgepoles will emerge as adult beetles next summer, flying off to attack more trees.

Bitter cold is likely the only way to break the cycle and keep the pine beetles from eating their way across the state.

But just how cold does it have to get, and for how long?

Even the experts, like Negron and Utah entomologist Barbara Bentz, say it’s hard to nail down exact numbers.

In fact, “How cold and for how long?” is one of the main questions that Negron, who works for the U.S. Forest Service, is trying to answer in the Fraser Experimental Forest. He’s been studying mountain pine beetles for three years at four Fraser sites.

Mountain pine beetle outbreaks surfaced in the 23,000-acre experimental forest in 2003. The problem steadily worsened, then exploded last summer, said site manager Mark Dixon.

Negron has temperature probes inserted a quarter inch beneath the bark of infested lodgepoles at four elevations. Other sensors monitor air temperature.

The tree and air temperatures are recorded every minute of every day. The numbers are stored on a battery-powered datalogger.

On a recent day, Negron trudged into his lowest-elevation site, at 9,000 feet, on snowshoes. He downloaded six months of temperature data into his laptop.

He said interior tree temperatures between minus-25 and minus-35 Fahrenheit are needed to kill mountain pine beetles. Once temperatures drop to that level inside the “phloem” layer where the larvae spend the winter, they’ll die within a day.

The problem is, the lodgepole’s thin bark provides a surprising amount of insulation from the cold. So it can take more than two weeks for killer cold to reach the larvae, he said.

And most of our intense cold snaps don’t last that long.

In addition, extreme cold events are becoming rarer and less severe than they used to be, said Bentz, a research entomologist with the federal Rocky Mountain Research Station in Logan.

“The winter temperatures are not as cold as they used to be, and some people think that is helping these insects be more successful,” Bentz said.

On Jan. 16, when the air temperature at Negron’s lowest-elevation site dropped to minus-24, it was three degrees warmer just a quarter inch beneath the bark.

Three degrees may not sound like much, but it can mean the difference between survival and the “supercooling point.”

That’s the temperature at which water inside the larvae freezes, killing them despite a natural antifreeze the bugs produce for winter protection.

In midwinter, the best beetle-killing scenario is an abrupt drop from relatively mild to bitterly cold temperatures, Bentz said.

If the temperature drops slowly and steadily, pine beetle larvae can adapt by gradually increasing production of glycerol, their alcohol-based antifreeze.

But if the drop is sudden and extreme, they may not have time to crank up glycerol production, she said.

“They’re not going to be able to acclimate properly,” she said. “There would be mortality.”

In recent decades, extreme cold snaps have become less common in Colorado’s north-central mountains, according to University of Colorado climatologist Klaus Wolter. He recently completed an analysis of Colorado temperature trends over the past 50 years.

During that time, the state’s average annual high temperature increased by about 1 degree Fahrenheit, while the average low temperature rose by 2 degrees.

North-central Colorado—the epicenter of the pine-beetle epidemic—warmed faster than any other part of the state during the past 50 years.

There, the average annual high temperature rose 2 degrees, and the average low temperature shot up more than 4 degrees, Wolter said.

“The fact that the minimum temperatures have gone up so much makes it much less likely that you’re going to get the cold needed to stop a big (insect) outbreak,” Wolter said.

Bentz said she plans to use a new computer model to study the likely effects of winter warming on mountain pine beetle populations in the West.

More in News

A member of a "sophisticated cocaine trafficking conspiracy" was convicted Monday in federal court in Denver of conspiring to distribute, and possessing with intent to distribute, five kilograms or more of cocaine, according to prosecutors.

A man who shot two eighth graders at Deer Creek Middle School in 2010, and was found not guilty by reason of insanity to attempted murder, will not be allowed to leave the Colorado Mental Health Institute's grounds without supervision, according to a Jefferson County District Court ruling.

After the San Francisco Bay Area, metro Denver experienced the biggest apartment rent increases this decade in the country. But plenty of new supply should put future rent gains closer to the national average, according to a new report from RealPage, a real estate research firm.